1. Field of the Invention
This invention relates to an over voltage and surge voltage preventing circuit for a display module.
2. Description of the Related Art
Generally, liquid crystal display (LCD) modules display pictures by controlling light transmittance of a liquid crystal with an electric field. To this end, LCD modules include a liquid crystal display panel containing liquid crystal cells arranged in a matrix pattern and driving circuits for driving the liquid crystal display panel.
In the liquid crystal display panel, gate lines and data lines cross each other. The liquid crystal cell is positioned at each area where the gate lines cross the data lines. An electric field is applied to each of the liquid crystal cells in the liquid crystal display panel using a plurality of pixel electrodes and a common electrode. Each pixel electrode is connected to any one of data lines, via source and drain electrodes of a thin film transistor as a switching device. Light transmittance within the LCD modules is controlled via an electric field applied between a pixel electrode and common electrode in accordance with a data voltage signal specific to each liquid crystal cell, thereby displaying a picture. However, when power is turned on, a surge voltage and an over voltage with an impulse shape are instantaneously generated and may damage driving circuits in the LCD module.
Referring to FIG. 1, an over voltage and surge voltage preventing device 4 for preventing surge voltages and over voltages is provided between a liquid crystal display module (LCM) 10 and a power supply 2.
In FIG. 1, the power supply 2 converts an externally applied alternating current (AC) voltage into direct current (DC) voltage capable of driving the LCM 10. The over voltage and surge voltage preventing device comprises a fuse 6 connected in series to a varistor 8 so as to prevent the application of any over voltage and surge voltage from the power supply 2 to the LCM 10. Conventionally, the fuse 6 is broken by heat generation when current exceeding a threshold value is applied from the power supply 2 thereby preventing an over current. The varistor 8 varies its resistance value non-linearly in accordance with a voltage variation applied from the power supply 2 to provide a constant voltage to the LCM 10.
However, in the over voltage preventing portion 8 of the conventional LCD module, the fuse effectively prevents over currents instead of over voltages, and the varistor is deleteriously sensitive to temperature increases such that varistors typically fail to completely prevent over voltages and surge voltages. Accordingly, when power is turned on in conventional LCD modules, over voltages and surge voltages from the power supply 2 are not reduced sufficiently and thus cause damage to driving circuits within the LCD module.